An FM (frequency modulation) system has been adopted as an exemplified modulation system in the BS (broadcasting satellite) analog broadcasting. In such a modulation system, while the receiving C/N ratio is about 10 dB or more, good receiving quality of pictures and sound is obtainable. However, once the receiving C/N ratio falls below about 10 dB, a truncation noise may be generated, which makes it impossible or difficult to obtain good receiving quality of pictures and sound. In the BS analog broadcasting, the receiving quality is degraded relatively moderately, as the receiving C/N ratio decreases.
On the other hand, in the BS digital broadcasting, once the receiving C/N ratio falls below a predetermined value due to rainfall or other reason, the receiving quality is drastically degraded, as compared with the BS analog broadcasting. In order to avoid such a drawback, a hierarchical modulation service has been provided in the BS digital broadcasting.
Specifically, in the BS digital broadcasting, at least seven modulation systems are available: 8PSK2/3, QPSK7/8, QPSK5/6, QPSK3/4, QPSK2/3, QPSK1/2, and BPSK1/2 modulation systems in Japan.
Among these, the bit rate of the 8PSK2/3 modulation system is the highest, and the transmission rate thereof is about 20 Mbps. The bit rate decreases stepwise in the order of the QPSK7/8, QPSK5/6, QPSK3/4, QPSK2/3, QPSK1/2, and BPSK1/2 modulation systems. The transmission rate of the BPSK1/2 modulation system having the lowest bit rate is about 2 Mbps.
In order to acquire high definition (HD) images in HDTV, a bit rate of about 20 Mbps is required to transmit MPEG-compressed data. In view of this, the 8PSK2/3 modulation system is adopted in the BS digital broadcasting of HDTV.
In the above modulation system, as far as the receiving C/N ratio is 9.5 dB or more, sufficiently high receiving quality is obtainable. However, once the receiving C/N ratio falls below about 9.5 dB, the receiving quality is drastically degraded, with the result that good receiving quality of pictures and sound is not obtainable. Specifically, whereas a required C/N ratio in frequency-modulation of the BS analog broadcasting is about 10 dB, a required C/N ratio in the 8PSK2/3 modulation system of the BS digital broadcasting is about 9.5 dB.
In other words, in the 8PSK2/3 modulation system of the BS digital broadcasting, once the receiving C/N ratio falls below about 9.5 dB, the receiving quality is drastically degraded. In view of this, a hierarchical modulation service has been provided to avoid such a drawback.
An example of the conventional digital TV broadcast receiver adaptive to the above hierarchical modulation service is disclosed in Japanese Patent No. 3,253,524. The publication discloses a receiver, wherein demodulation processes respectively suitable for upper layer information and lower layer information are implemented, and signal receiving is switched over between the upper layer information and the lower layer information.
The above hierarchical modulation service has been provided to transmit a video signal (also called as lower layer information) which exclusively carries video composing a base of a program and is stably receivable even in a weak signal receiving state, along with a video signal of a high quality (also called as upper layer information) with respect to each program.
The 8PSK2/3 modulation system is adopted for the upper layer information transmission, and the BPSK1/2 modulation system is adopted for the lower layer information transmission, for instance. Although the BPSK1/2 modulation system cannot assure high quality image display due to its slow transmission rate, the modulation system enables to maintain a lowest viewable receiving quality until the receiving C/N ratio becomes substantially 0 dB.
The receiver is configured to switch over signal receiving between the upper layer information and the lower layer information which are simultaneously transmitted from a broadcasting satellite, depending on the receiving C/N ratio, for allowing viewers to watch a program based on the upper layer information or the lower layer information. Specifically, if the receiving C/N ratio is 9.5 dB or more, the receiver receives the upper layer information that has been modulated according to the 8PSK2/3 modulation system, and decodes the modulated signals for allowing the viewers to watch the program based on the upper layer information. On the other hand, if the receiving C/N ratio falls below about 9.5 dB, the receiver stops receiving the upper layer information and starts receiving the lower layer information that has been modulated according to the BPSK1/2 modulation system and decodes the modulated signals for allowing the viewers to watch the program based on the lower layer information.
Japanese Unexamined Patent Publication No. 2002-009854 discloses a receiver adaptive to the hierarchical modulation service, as an example of the conventional BS digital TV receiver for receiving the upper layer information and the lower layer information. FIG. 8 is a block diagram showing primary components of the BS digital TV receiver.
As shown in FIG. 8, a broadcast stream BCS including waves modulated in channels BS1 through BS15 is received by an antenna (not shown), a channel is selected from among the channels BS1 through BS15 by a tuner 912, and the transport stream (TS) is packetized by a demodulator/decoder 914. A transport stream packet (TSP) processor 918 selects service packets from the transport stream packets, and separates video data packets from audio data packets. A video decoder 922 (including an MPEG decoder) receives the video data packets selected by the TSP processor 918 and a D/A converter 924 outputs a video output signal VS. Likewise, an audio decoder 923 receives the audio data packets selected by the TSP processor 918 and a D/A converter 925 outputs an audio output signal AS.
In the BS digital broadcasting, there is a case that a conditional access system is adopted. According to the conditional access system, specific users who have contracted to pay the receiving fee are authorized to watch a certain program or programs. In such a case, the broadcast stream BCS includes scrambled broadcast program data and specific information (including a key to decrypt the scrambled data). A controller 916 is connected to an IC card circuit or a like device for processing the specific information carried by the received broadcast stream BCS.
Data representing the output error rate of the upper layer information and the output error rate of the lower layer information are detected by the demodulator/decoder 914, and the controller 916 constantly monitors the respective values of the data (e.g., every 10 ms). Information regarding judgment as to whether the hierarchical modulation service is carried out is recorded in a program map table (PMT) defined by MPEG. The TSP processor 918 notifies the controller 916 of the PMT in decoding of the PMT.
The demodulator/decoder 914 has a demodulation circuit, a Viterbi error correction circuit, and a Reed-Solomon error correction circuit. The demodulation circuit demodulates a modulated digital signal whose frequency has been converted by the tuner 912 to a base band signal. After the Viterbi error correction circuit and the Reed-Solomon error correction circuit respectively implement their error corrections with respect to the base band signal demodulated by the demodulation circuit, the signal is decoded and outputted to the TSP processor 918.
The bit error rate (hereinafter, simply called as “BER”) of the data is detectable by the demodulation circuit, the Viterbi error correction circuit, and the Reed-Solomon error correction circuit. For instance, if the BER detected by the demodulation circuit is 0.01, the BER detected by the Viterbi error correction circuit is 0.0001, and the BER detected by the Reed-Solomon error correction circuit is 0.00000001. Thus, the bit error rate decreases as the error correction is implemented by the demodulation circuit, the Viterbi error correction circuit, and the Reed-Solomon error correction circuit successively in this order.
The finally obtained bit error rate is sent from the demodulator/decoder 914 to the controller 916, which, in turn, selects the upper layer information or the lower layer information based on the bit error rate, and notifies the TSP processor 918 of the selection result. The TSP processor 918 selects video/audio data packets of the upper layer information, or video/audio data packets of the lower layer information, based on the notification sent from the controller 916, and sends the selected video data packets to the video decoder 922 and the selected audio data packets to the audio decoder 923, respectively. Thus, even in a case where the receiving signal level is lowered due to rainfall or other reason, at least the lower layer information is receivable for allowing viewers to watch a program based on the lower layer information.
In the above prior art technology, since switching between the upper layer information and the lower layer information is implemented based on a judgment as to whether the received signal level is below or above the predetermined level, a low-quality image based on the lower layer information is resultantly displayed on the entirety of a TV screen, despite the fact that all the image data constituting the entirety of the screen image are not necessarily in a error condition.
Further, it is a general practice to apply a so-called hysteresis to a switching operation between the upper-layer-information-based image display and the lower-layer-information-based image display in order to prevent generation of flicker on an on-screen image arising from frequent switchover between the upper-layer-information-based image display and the lower-layer-information-based image display. This is a function of keeping the upper-layer-information-based image display from being resumed as long as the receiving signal level is not recovered to a predetermined desirable level, once the switchover from the upper-layer-information-based image display to the lower-layer-information-based image display has been done.
Providing the above function makes it possible to prevent generation of flicker on an on-screen image arising from frequent switchover between the upper-layer-information-based image display and the lower-layer-information-based image display. However, in the above arrangement, viewers are forced to watch a program of a low-quality image for a long time despite a possibility that the upper layer information may be receivable during the low-quality image display period.